The present invention relates to controlling a high-precision positioning system. More particularly, the present invention relates to achieving a desired magnification and orthogonality in a scanning microlithographic system.
High-precision positioning systems, such as those used in machining tools or lithography, drive components of the system in accordance with a defined path within narrow tolerances. The path may have one direction, e.g., the X coordinate, or two directions, e.g., the X and, orthogonal Y coordinates. Typically in a scanning microlithography apparatus for semiconductor wafer processing, a stage is used to position a mask (reticle) in two dimensions, while a separate stage is used to position the plate (wafer). The stages are both carried upon a coarse stage that moves at a constant velocity during exposure of the plate. The entire system is supported by a base structure along with other components such as a source of radiant energy and a projection lens to focus the energy.
FIGS. 1 and 2 show a respective front view and perspective view of a simplified scanning microlithographic positioning table mechanism 10. Table mechanism 10 includes a coarse stage 12; a mask fine stage 14, which holds a mask (reticle) 15; and a plate fine stage 16, which holds a plate (wafer) 17. FIG. 1 also shows a projection lens 18 and an illumination apparatus 20, which produces a light beam 21 for exposing plate 17. Coarse stage 12 is supported by a base structure (not shown) containing an antivibration device and anti-friction bearings, such as air bearings or roller bearings, as is well known in the art. Coarse stage 12 is conventionally driven at a constant velocity in the X coordinate direction, as illustrated by arrow 13 in FIG. 2, during exposure of plate 17. Illumination apparatus 20 and projection lens 18 remain relatively stationary.
Mask fine stage 14 and plate fine stage 16, which are attached to coarse stage 12, move at the same velocity during exposure of plate 17. Thus, in theory table mechanism 10 has a magnification of 1:1, i.e., the area of mask 15 that is scanned across illumination apparatus 20 is equal to the area of plate that is exposed. Additionally, because mask fine stage 14 and plate fine stage 16 are attached to coarse stage 12 and move at the same velocity, the magnification factor and orthogonality factor of table mechanism 10 are fixed.
In practice, however, there may be error in the magnification caused by projection lens 18 or synchronous error caused by the motion of coarse stage 12. Additionally, the projection lens 18 and synchronous error may create error in the orthogonality of the exposed image. Orthogonality error results in the image from mask 15 being reproduced on plate 17 in a skewed manner (an example of orthogonality error is shown in FIG. 5B). These errors result in poor quality and low resolution of the exposed image. Thus, it is advantageous to correct magnification and orthogonality error in the positioning system or change the magnification factor or orthogonality factor to a desired value.
A scanning positioning system in accordance with the present invention moves one fine stage relative to the another fine stage to correct magnification and orthogonality error or to change the magnification or orthogonality factors to a desired level. A scanning position system, such as a microlithographic system, has a mask fine stage and a plate fine stage mounted on a coarse stage that moves at a constant velocity during the exposure of the plate. The position of one fine stage is measured relative to the other fine stage. With this position information, one of the fine stages may be moved by an appropriate amount in the same or opposite direction of the scan to increase or decrease the magnification. Thus, the scanning position system can change the magnification factor to a desired level, as well as a correct any magnification error or any rotation error. Likewise, by moving one of the fine stages by an appropriate amount in a direction perpendicular to the scan, the system can change the orthogonality factor by a desired amount, as well as correct any orthogonality error.
In one embodiment of the present invention, the mask fine stage has a mask frame attached to the coarse stage. A mask holder is attached to the mask frame via actuators that move the mask holder relative to the mask frame and the coarse stage. A position control apparatus receives information regarding the position of the mask fine stage relative to the plate fine stage. Based on this position information, the position control apparatus sends commands to the actuators to move the mask holder relative to the plate fine stage by an amount necessary to achieve the desired magnification and orthogonality. The position control apparatus can also be used to simultaneously compensate for any synchronous error between the fine stages and reduce the settling time of the system.